1. Field of the Invention
The present invention relates to a circuit board with a circuit pattern formed thereon and coated with a resist film for protecting the surface thereof.
2. Description of the Related Art
Usually, a circuit board is provided with a circuit pattern formed thereon and a protective resist film is coated on the surface thereof to prevent oxidation of the circuit pattern. The resist layer is a thin polymer coating designed to prevent both oxidation and shorting of the circuit pattern. Oxidation could raise the resistance of the exposed circuit line significantly, or cause the circuit line to break completely, creating an open circuit condition. At the same time, due to the proximity of adjacent circuit lines, shorting between the circuit lines could arise from contamination by environmental debris. Further, the resist layer has no durability against environmental damage.
In a conventional circuit board, however, when another unit is brought into contact with the resist film, the resist film is easily damaged by the impact with the other unit, and as a result, the circuit pattern formed on the board is directly exposed to the outside air and is oxidized, or is broken by the impact of the contact with the other unit. When the other unit brought into contact with the resist film has a temperature of 70.degree. C. or more, the resist film may be melted by the heat radiated by the other unit. Furthermore, it is possible that the resist film will absorb water when used in humid or damp environments, and thus become soft. In this case, the resist film is more readily damaged by the other unit, and is easily peeled from the circuit pattern.
The circuit board as described above is generally used for a keyboard, electronic equipment or the like (as disclosed in Japanese Unexamined Patent Publication No. 63-20613 laid open on Jan. 28, 1988, which corresponds to U.S. patent continuation application Ser. No. 07/489,874 filed on Mar. 6, 1990 and assigned to the same assignee as that of the present application, which is a continuation of patent application Ser. No. 07/072,400 filed on Jul. 13, 1987 and now abandoned) and thus a circuit board having a greater durability and reliability is required.